When David was old and stricken in years, so the First Book of Kings tells us, his servants brought him a young virgin, Abishag the Shunamite, to "lie in his bosom so that our lord the king may get heat". "And," the chronicle continues, "the damsel was very fair, and cherished the king, and ministered to him: but the king knew her not." Transposed to the 18th century, this warming tale applies to the ageing Horace Walpole, who had two Abishags, sisters Mary and Agnes Berry.

Walpole's delicious letters to the Misses Berry were written during the last eight years of life, when he was an old man - very old for the day: he was in his 70s - but still full of charm and intellectual vigour.

The Berry sisters were his delight; he was more than half in love with them, and stopped himself proposing to Mary, the elder, only by thinking what people would say if a lifelong bachelor espoused a girl in her 20s. "When an ancient gen tleman marries," he wrote, "it is his best excuse, that he wants a nurse; which I suppose was the motive of Solomon, who was the wisest of mortals... for marrying a thousand wives in his old age, when I conclude he was very gouty. I in humble imitation of that sapient king, and no mines of Ophir flowing into my exchequer, espoused a couple of helpmates."

Despite appearances, the friendship was not unequal. Walpole was rich and famous - and the scion of a house made great by his prime minister father, Sir Robert Walpole - whereas the Berrys were poor and socially obscure. He patronised them, in the best sense of the term; he gave them a house in the grounds of his Gothic creation, Strawberry Hill, and left them well provided in his will.

But the sisters had much to offer. They spoke French fluently, and read Latin. They had travelled, and knew France and Italy almost as well as Walpole. They were cultured, intelligent, unpretentious and affectionate. After Walpole's death Mary served as his literary executor, producing the first complete edition of his works. Her journals and letters are one of the chief sources of information about his life.

Walpole's letters to the Berrys are not just valuable but perhaps essential to our understanding of him. He is an important figure in the intellectual history of 18th- century England; his celebrated Gothic novel, The Castle of Otranto - highly praised by Sir Walter Scott and damned by Hazlitt - is the start of English romanticism. His memoirs of political life are an invaluable historical document, for although he was inveterately hostile to his father's political enemies, he was fair in reporting parliamentary debates and personalities, of which- as a silently observing MP for over a quarter of a century - he had intimate knowledge.

But Walpole's chief contribution to literature is the epistle. He passionately admired the letters of the 17th-century Madame de Sévigny, who had revived the classical tradition of letter-writing as a civilised accomplishment. In more than 3,000 letters spanning 60 years Walpole took the epistolary art to new heights.

His chief correspondents included Sir Horace Mann, British resident of Italy, the Milton scholar William Cole, the Countess of Ossory, his old friend Henry Seymour Conway, and the Parisian salon hostess, blind Madame du Deffand (who, when told of the miracle of St Denis - alleged to have picked up his decapitated head and walked 200 km to Paris - tartly remarked, "Well, in such cases only the first step is difficult.")

But in his letters to the Berry sisters Walpole is at his most domestic, even at times at his most vulnerable, as when the sisters made a channel-crossing on a night of storms, and Walpole haunted the gloom of Strawberry Hill in terror for their lives. The letters are unbuttoned, fond, playful, gallant and doting. They glance at great events of the day - the trial of Warren Hastings, the storming of the Bastille and the early days of the French Revolution - giving them an extraordinary freshness and immediacy, reminding one how sharp history's realities were for contemporaries.

And in return we see the life of the Misses Berry - Agnes's aspirations in painting and Mary's crushing disappointments in love, reflected in the mirror of Walpole's concern. In all their lightness Walpole's letters are a human testament, witness to what he calls in one of them "the grace of friendship" - words that the editor of his letters aptly used as their title.